1. Field of the Invention
The present invention relates to a liquid-repellent film forming method, an inkjet head and an inkjet recording apparatus, and more particularly, to technology for forming a liquid-repellent film having liquid-repellent properties on a surface of a nozzle plate having nozzle apertures for ejecting liquid droplets.
2. Description of the Related Art
In a recording head (also referred to as an inkjet head) used in an inkjet recording apparatus, droplets of ink are ejected through nozzles having nozzle apertures in the surface of a nozzle plate, which constitutes an ink ejection surface of the recording head. If the ink has adhered to the surface of the nozzle plate (and in particular, to the periphery of the nozzle apertures), ink droplets subsequently ejected from the nozzles are affected and ejection instabilities occur, for instance, variations arise in the ejection direction of the ink droplets, giving rise to a deterioration in image quality. Therefore, in order to prevent these problems, the surface of the nozzle plate on the ejection side of the nozzle apertures (hereinafter referred to as an “ejection side surface”) is coated with a liquid-repellent film.
Various different technologies have been proposed hitherto for forming a liquid-repellent film on the ejection side surface of a nozzle plate.
For example, Japanese Patent Application Publication No. 2003-286478 discloses a liquid-repellent film formed on a nozzle plate and a method of manufacturing same. The liquid-repellent film contains molecules having at least one siloxane bond (—Si—O—) at either end and including a vinyl group and/or an ethynyl group and a benzene ring in the intermediate part, and molecules having a carbon fluoride chain at one end and at least one siloxane bond at the other end, which the two types of molecules together form a polymer. It is considered possible to thereby form the liquid-repellent film having alkali resistant properties.
Japanese Patent Application Publication No. 2009-029068 discloses a method of manufacturing a nozzle plate for a liquid ejection head made of silicon. The nozzle plate is formed with nozzles through which the liquid is ejected, and has a liquid-repellent film on a surface where apertures of the nozzles are arranged. In the manufacturing method, the following steps are carried out successively: a step of preparing a silicon substrate having a silicon oxide film on the surface where the nozzle apertures are arranged; a step of carrying out a chemical activation process for activating the silicon oxide film by removing the surface by chemical reaction in a dry process; a step of carrying out a physical activation process for activating the silicon oxide film by removing the surface by physical breakdown in a dry process; and a step of arranging the liquid-repellent film on the silicon oxide film. It is considered possible to thereby form the liquid-repellent film having durability on the surface where the nozzle apertures are arranged.
Japanese Patent Application Publication No. 2003-286478 is directed to technology for improving the material properties of the liquid-repellent film and improving the durability with respect to alkaline solutions, and Japanese Patent Application Publication No. 2009-029068 is directed to technology for strengthening the bond between the liquid-repellent film and the underlying silicon oxide film by cleaning and activating the surface of the silicon oxide film through carrying out a plasma treatment on the silicon oxide film to remove organic material.
Japanese Patent Application Publication No. 2008-105231 discloses a liquid-repellent film forming method which forms a liquid-repellent film having liquid-repellent properties on the surface of a nozzle plate. The method includes: a first step of forming an underlying layer composed of a plasma polymerization film, on one surface of a nozzle plate; a second step of carrying out an oxidization process on the surface of the underlying layer in a gas atmosphere of a dew point at −40° C. to 20° C., and introducing a hydroxyl group and/or adsorption water; and a third step of forming the liquid-repellent film on the underlying layer on which the oxidization process has been carried out. It is considered possible to thereby form the liquid-repellent film having improved adhesion to the underlying layer and resistance to wear.
However, even if the material properties of the liquid-repellent film are improved as in Japanese Patent Application Publication No. 2003-286478, there is a drawback in that if the processing of the underlying layer is incomplete, then sufficient bonding sites (hydroxyl groups: OH groups) are not created. Then, the bonding between the liquid-repellent film and the underlying layer is not sufficient, and the film properties are declined. Moreover, even if cleaning and surface activation is carried out by plasma treatment of the underlying layer only as in Japanese Patent Application Publication No. 2009-029068, there is a drawback in that sufficient reaction sites are not created on the surface and a high-density liquid-repellent film having sufficient resistance to alkalis is not obtained.
Over and above the aforementioned drawbacks, a problem that is common to the technologies in Japanese Patent Application Publication Nos. 2003-286478 and 2009-029068 is the fact that the liquid-repellent film is bonded to the nozzle plate through siloxane bonds, which are liable to be hydrolyzed in liquids containing OH groups, such as water or aqueous solutions containing alkalis. Consequently, if alkaline ink droplets, for example, are ejected through the nozzle apertures of the nozzle plate, as in the inkjet head, then the liquid-repellent film in the related art is liable to be erased by contact with the ink, and it is thus not possible to improve resistance to alkalis.
Furthermore, there is also technology which improves film density by increasing the film thickness, and increases the overall lifespan of the film, even if deterioration occurs, as in the plasma polymerization film in Japanese Patent Application Publication No. 2008-105231; however, this leads to increase in raw material quantity and costs, and moreover, since the film is essentially based on siloxane network, then it is also not possible to improve resistance to alkalis.